1. Field of the Invention
The present invention relates generally to drilling rigs and, more particularly, to a self-propelled well servicing and workover rig for use in harsh arctic environments.
2. Description Of The Related Art
When working in harsh arctic environments, even the simplest tasks which must be performed outside often become quite difficult. For instance, during the winter months in Alaska on the North Slope, ambient temperatures often fall below -50.degree. F. Since oil exploration and production continues throughout the winter in these areas, workmen working outside are subjected to the snow, wind and bitter cold. As a result, the productivity and morale of the workmen suffer in these less than ideal working conditions.
To mitigate these problems heated shelters are provided where possible. Typically, these shelters must surround cumbersome machinery and still provide enough space for the workmen to operate and maintain the machinery. Although providing shelter at a fixed work site can be accomplished by building a semi-permanent structure, providing shelter on temporary work sites poses greater problems. For instance, wells may require workovers when production of the oil slows or ceases. Workovers may include through-tubing clean outs, stimulations, and fishing jobs.
Workovers can be accomplished by erecting a freestanding derrick over the well, performing the workover, and then disassembling the derrick. However, workovers typically require much less time to complete than the initial drilling operation. Therefore, assembling and disassembling a derrick to perform a workover introduces an inefficiency that may significantly offset the benefits of the workover. To overcome this inefficiency, portable drilling rigs, commonly called "workover rigs," typically perform workovers. These portable drilling rigs include a derrick that is mounted on a trailer or self-propelled chassis. The derrick may be pivoted from a portable position, where the derrick rests horizontally on the rig, to a working position, where the derrick is fixed in a substantially upright position over the well.
Workovers require much equipment in addition to the portable derrick. Many downhole operations are accomplished using a pipe string that is introduced into the well from the derrick. The pipe string is made up of a series of short interconnected pipe sections. In a drilling operation, for instance, a bit is placed on the lower end of the pipe string, and the pipe string is rotated from above the earth's surface by a suitable rotary drive mechanism. As the bit bores deeper, additional pipe sections must be connected to the pipe string. Moreover, in order to satisfactorily form the bore, the cuttings produced as the bit bores deeper must be carried out of the well hole. For this purpose, a mud slurry is pumped downwardly through the pipe to gather the cuttings, and then pumped upwardly around the annulus between the pipe and the well hole to remove the cuttings. Therefore, a supply of tools, pipe, and mud should be readily available to the drillers performing the workover.
Supplying tools, pipe, and mud to a portable drilling rig in warm climates presents relatively few problems. The pipe is simply arranged in racks adjacent the derrick. As more pipe is needed, operators load pipe onto a suitable conveyor and guide the pipe toward the derrick along a ramp, typically referred to as a beaver slide, for delivery to the derrick. Similarly, equipment for mixing a suitable slurry of mud may be positioned outside on the ground adjacent the derrick. Then, it is a relatively simple procedure for operators to arrange pumps and piping for pumping mud into the well through the drill string and out of the well through the well annulus. Moreover, when changing a drill bit, or withdrawing pipe from the well for any other reason, the detached sections of pipe may be simply removed from the derrick and placed in a convenient location on racks on the ground.
However, portable drilling rigs of this type present distinct disadvantages when used in a cold environment. For example, the rigs offer no shelter to the workmen. Furthermore, the pipe may become damaged or difficult to manage because it is not protected from the environment. In frigid environments, ice may form on the threads of pipe stored outside and, thus, cause the threads to deform during connection unless the ice is removed. To overcome these problems, winterized portable drilling rigs have been developed. On these winterized rigs, portions of the chassis carrying equipment for use by the workmen are enclosed in a housing which may be thought of as a vehicle body.
In addition to the winterizing of the portable derricks themselves, pipe shelters and mud/utility support modules have been developed. A pipe shelter essentially includes a trailer having a number of pipe racks for carrying the pipe to be used on the derrick, and a pipe conveyor for transferring pipe up the beaver slide to the derrick. These apparatus are enclosed by a winterized housing which has a door therein for receiving additional loads of pipe. In cold climates, it is important to keep the pipe warm so that new sections of pipe easily interconnect with pipe in the pipe string.
In a similar fashion, the mud module includes the necessary mud producing equipment, such as water and mud tanks, which is mounted on a trailer chassis. A winterized housing encloses the equipment on the trailer chassis, and usually includes a passageway for interconnecting the mud module to the portable drilling module. Thus, once the mud module and the pipe shelter are interconnected with the portable drilling module, the winterized drilling rig protects the workmen and the necessary support equipment from the environment.
Unfortunately, these winterized rigs still exhibit certain disadvantages. For example, at -50.degree.F., it is important that the winterized shelters of the various modules of the portable drilling rig protect the workmen in as many phases of normal operation as possible. As one example, a pipe shelter includes a door for loading and unloading pipe. If this door remains open too long, the heat within the shelter will rapidly escape, thus leaving the workmen inside exposed to frigid temperatures.
As another example, it is advantageous if the workmen can retrieve pipe from the well bore quickly and efficiently. As sections of pipe are detached from the drill string, the pipe must be stored, at least temporarily. Therefore, many times workmen will open a door in the derrick housing and place detached sections of pipe on the ground outside of the derrick. Of course, every time the derrick is opened, heat escapes and the workmen are exposed to the cold environment. Additionally, workmen are exposed when they leave the derrick to collect the pipe. Alternatively, detached sections of pipe may be sent down the beaver slide into the pipe shelter. While this alleviates the problem of exposing workers to the cold environment, it is a slow and inefficient way of temporarily storing pipe.
In addition to not properly sheltering the workmen during all phases of an operation, many winterized rigs cannot be used as the weather warms. In the winter in Alaska, the ground remains frozen, and the heavy winterized rigs travel easily from one work site to another. However, in the summer months, the ground softens. In some instances, the rigs face the danger of becoming stuck if the ground becomes too soft. In any event, in the summer months the rigs tend to damage roadways and, therefore, become undesirable.
Moreover, self-propelled rigs are much preferred as compared to trailer-mounted rigs. Trailer-mounted rigs must be hauled from one worksite to another by large tractors. Not surprisingly, these tractors are quite expensive to operate, lease and own. In addition, self-propelled rigs can be easily configured for transportation so that they are easier to move than trailer-mounted rigs.
The present invention is directed to overcoming, or at least minimizing, one or more of the problems as set forth above.